Note: the following is generally a bad idea.
You could always limit the number of pseudoterminals. sshd can only start a shell for a user if there is an available pseudoterminal "device". The max number is set with a sysctl. To see what it's currently set to, use
cat /proc/sys/kernel/pty/max
To change it short-term, you can use something like this:
echo "10" > /proc/sys/kernel/pty/max
To make it persist across reboots, you would have to set it in your sysctl.conf using the standard syntax.
Again, doing this is probably a bad idea. If all the pts devices get consumed, you won't be able to open a session to raise the limit, so you could be locked out. Be absolutely sure you are willing to accept the risk of not being able to fix something quickly when it breaks.
If it goes badly wrong, you should still be able to boot into single-user mode, mount the drive, remove the line from your sysctl.conf, and reboot. This requires hands on the system or a good LOM card (and I wouldn't call Check Point's LOM cards "good").